This invention relates to ammonia oxidation. Ammonia oxidation is widely employed in the manufacture of nitric acid and hydrogen cyanide. In the manufacture of nitric acid ammonia is oxidised with air to nitric oxide, while in the manufacture of hydrogen cyanide a mixture of ammonia and methane (often as natural gas) is oxidised with air. In both processes, the gas mixture is passed at an elevated temperature over a catalyst to effect the oxidation. Side reactions, such as the formation of nitrogen or nitrous oxide, are undesirable. Consequently, in addition to good activity, the catalyst is required to have a good selectivity.
For many years the catalysts employed have been platinum sometimes alloyed with other precious metals, in the form of meshes or gauzes formed from the metal wire. Such catalysts have good activity and selectivity but suffer from the disadvantage that not only is the catalyst very expensive, but at the temperatures encountered, the metals exhibit an appreciable volatility and so gradually the metal is lost into the gas stream. While it is well known to provide downstream means to trap the volatilized metal so that it may be revered subsequently, because of the continual volatilization, the life of the catalyst is short and frequent replacement is necessary. Furthermore the recovery of the metals from the downstream trap and re-fabrication of the catalyst meshes or gauzes involves a considerable allocation of working capital.
It is therefore desirable to provide a replacement for such precious metal catalysts.
It is well known that cobalt oxide exhibits activity for ammonia oxidation. In order to improve the activity and selectivity there have been numerous proposals to incorporate various promoters such as rare earths into a cobalt oxide catalyst.
For example, it has been proposed in CN-A-86108985 to employ lanthana/ceria/cobalt oxide compositions of the general formula La1-xCexCoO3 (where x is from 0 to 1) made by a specified co-precipitation route as ammonia oxidation catalysts. Such materials were reported as having good activity and selectivity when tested on a small scale, although there is some suggestion that the activity and/or selectivity is decreased at operating temperatures in the upper end of the temperature range normally employed for ammonia oxidation (800-1000xc2x0 C.).
We have found that it is important that, in this type of catalyst, the bulk of the cobalt is present as a mixed oxide phase, e.g. as the Perovskite structure RECoO3, (RE=rare earth), or a form thereof in which the oxygen is non-stoichiometric, and is not present as free cobalt oxides e.g. cobalto-cobaltic oxide CoO3O4 or cobalt monoxide CoO. We believe that it an appreciable proportion of the cobalt is present as the free oxides; in use at high temperatures, e.g. above about 850xc2x0 C., the free cobalt oxides are able to catalyse the side reaction oxidations e.g. to nitrogen or nitrous oxide, whereas if the bulk of the cobalt is xe2x80x9clockedxe2x80x9d into a mixed oxide phase, such as the Perovskite structure, the oxidation capability is more limited to the desired oxidation.
Producing the catalyst simply by co-precipitation of the component oxides (or compounds that readily decompose thereto) or by evaporating a solution of a mixture of thermally decomposable salts, e.g. nitrates, of the desired metals, followed by calcination at moderate temperatures, e.g. 600-900xc2x0 C., does not necessarily lock the bulk of the cobalt into a mixed oxide phase such as the Perovskite structure even if the components are present in the requisite proportions. Heat treatment of the product is necessary to obtain the desired structure. In the aforesaid CN-A-86108985 the catalysts were calcined at 900xc2x0 C. for 5 hours prior to use; we believe that such heat treatment is inadequate, and treatment at higher temperatures and/or for longer times is required to minimise the quantity of free cobalt oxide present. However heating at too high a temperature, above about 1150xc2x0 C., may give rise to decomposition of mixed oxide phases, releasing free cobalt oxides. Alternatively, or additionally, steps may be taken to remove free cobalt oxides from the composition: for example the composition may be washed with an ammoniacal solution or other solution containing a complexing agent for cobalt. Ethylene diamine tetra-acetic acid is an example of such a complexing agent.
Accordingly the present invention provides an oxidation catalyst comprising oxides of (a) at least one element A selected from rare earths and yttrium, and (b) cobalt, said cobalt and element A begin such proportions that the element A to cobalt atomic ratio is in the range 0.8 to 1.2, at least some of said cobalt and element A oxides being present as a mixed oxide phase with less than 30%, preferably less than 25%, of the cobalt (by atoms) being present as free cobalt oxides.
The catalyst thus contains at least one mixed oxide phase containing cobalt and at least one element A. The catalyst may also contain free element A oxides and/or one or more mixed oxide phases containing two or more elements A. The element A to cobalt atomic ratio is 0.8 to 1.2, particularly 1.0 to 1.2. Preferably less than 25% (by atoms) of the cobalt is present as free cobalt oxides, and in particular it is preferred that less than 15% (by atoms) of the cobalt is present as the cobalt monoxide, CoO. The proportion of the various phases may be determined by X-ray diffraction (XRF) or by thermogravimetric analysis (TGA) making use, in the latter case, of the weight loss associated with the characteristic thermal decomposition of Co3O4 which occurs at approximately 930xc2x0 C. in air. Preferably than 10%, particularly less than 5%, by weight of the composition is free cobalto-cobaltic oxide end less than 2% by weight is free cobalt monoxide.
Preferably at least one element selected from yttrium, cerium, lanthanum, neodymium, and praseodymium is used as part or all of element A. Element A may comprise a mixture of at least one variable valency element Vv selected from cerium and praseodymium and at least one non-variable valency element Vn selected from yttrium and the non-variable valency rare earth elements such as lanthanum or neodymium. In particular it is preferred that the atomic proportions of variable valency element Vv to non-variable valency element Vn is in the range 0 to 1, party 0 to 0.3. It is preferred that most of the cobalt is present as a Perovskite phase ACoO3, but where element A comprises two or more elements, e.g. Vv and Vn, it is not necessary that there is a mixed Perovskite phase, e.g. VvxVn1-xCoO3 where x is between 0 and 1. Thus there may be a Perovskite phase, e.g. VnCoO3 or VvCoO3, mixed with other phases such as Vv2O3, Vn2O3, (VvxVn1-x)2O3 or VvxVn1-xO2.
As indicated above the catalyst may be in a form wherein the amount of oxygen is non-stoichiometric. This arises from the variable valency of cobalt and also of any variable valency rare earth present as part, or all, of element A.
The catalyst may be formed by heating a composition containing the cobalt and element A oxides, preferably in air, to a temperature in the range 900-1200xc2x0 C. in order to produce a material in which only a small proportion of the cobalt is present as free oxides.
The compositions may be made by precipitation, e.g. by adding a solution of soluble salts of the relevant metals to a solution of a base, e.g. ammonium carbonate or hydroxide, to precipitate the relevant metals as (basic) carbonates, hydroxides, or oxides followed by calcination to convert the precipitated compounds to the oxides. The use of alkali metal compounds as the base to effect precipitation is less preferred as they inevitably cause some contamination of the product with sodium which could act as a catalyst poison. The precipitation may alternatively, but less preferably, be effected by adding the base to the solution of the mixed salts. Alternatively, the composition may be made by forming a solution of thermally decomposable salts, e.g. nitrates or salts of organic acids, e.g. oxalates or citrates, of the metals in the appropriate proportions and evaporating the solution to dryness followed by calcination to effect decomposition to the appropriate oxides. Less preferably, the composition may be made by mixing preformed oxides of the metals in the appropriate proportions.
In another alternative sore or all of the element A material may be used as a support on to which the cobalt and any remaining element A is coated. Thus a finely divided element A oxide, e.g. ceria, may be impregnated with a solution containing a cobalt salt and possibly also an element A salt, e.g. a lanthanum salt, followed by decomposition of the cobalt and any element A salts. Alternatively, such a supported material may be made by precipitation by precipitating the cobalt and optionally some of the element A, as heat decomposable compounds on to a finely divided, e.g. precipitated, element A oxide or compound decomposable thereto.
Whichever route is used to make the oxides composition, the composition should be calcined. e.g. in air, at a high enough temperature for long enough to form sufficient material with a mixed oxide structure, e.g. the Perovskite structure, to combine most, if not essentially all, of the free cobalt oxides into one or more mixed oxide phases. As indicated above the calcination temperature is preferably in the range 900-1200xc2x0 C. The duration of the heating required will depend on the temperature employed and on the route employed to make the composition. If the heating temperature is below 1100xc2x0 C., heating for at last 6 hours is preferred. On the other hand the duration of heating at a temperature above 1150xc2x0 C. is preferably less than 6 hours in order to minimise the decomposition of cobalt oxide containing phases into free cobalt monoxide. However catalysts prepared by evaporating a solution containing a mixture of organic salts, e.g. citrates, of the relevant metals to dryness followed by calcination may require heat treatment for shorter times and/or at temperature 200-300xc2x0 C. below the temperatures required for compositions made for example by precipitation. On the other hand if the catalyst is made by calcining a mixture of preformed oxides, longer times and/or higher temperatures may be required to produce a material in which only a small proportion of the cobalt is present as free oxides.
In the aforesaid CN-A-86108985 the catalysts were tested on a small scale with the catalysts in the form of a bed of a coarse powder.
For practical reasons it is not desirable to employ a bed of a powdered catalyst in a full size ammonia oxidation plant: desirably the catalyst should be in a form such that it is a direct replacement for the precious metal meshes or gauzes conventionally used.
It has been proposed in Czech patent CS 266106 to employ a catalyst in the form of a stainless steel mesh bearing a coating of a mixture of cobalt oxide promoted with small amounts of ceria, chromia and/or alumina. However, such catalysts, which contain far more cobalt than is required for the Perovskite structure, and will inevitably contain substantial proportions of free cobalt oxides.
In order to generate adequate catalyst surface area when using a wire support it is necessary to give the support a ceramic coating, termed a wash coat, and the active material is then deposited on this wash coat. Normally alumina or lanthana compositions are used as such wash coats. However, with conventional high temperature steel supports, there is a risk that material of the wash coat, or impurities remaining therein, e.g. alkali, resulting from the use of alkali aluminate solutions to form the wash coat, may in use gradually diffuse into the active material, upsetting the desired structure, and interfering with the catalytic performance.
We have found however that by the use of primary supports made from a high temperature, aluminum containing, ferritic alloy, it is possible to obtain good adhesion of the wash coat to the primary support without the use of alkaline wash coat solutions and so the problem of migration of alkali impurities into the active catalysts may also be avoided.
Catalytic processes using a bed of random packed catalyst support units having a plurality of through passages and on which the catalyst is supported and in which the support units may be made from such alloys have been proposed in GB-A-2077136. That reference lists ammonia oxidation as an example of a catalytic process for which such units may be used. That reference also refers to GB-1568861 for methods of applying a suitable wash coat which does not involve the use of alkaline wash coat solutions.
Suitable iron/aluminium alloys are those described in the aforesaid GB-A-2077136, and in particular are those of the weight composition
The presence of yttrium and/or cerium is preferred as these exert a stabilising effect on the alumina formed upon calcining the alloy or final catalyst. The presence of cobalt may also be desirable to minimise migration of components from the alloy or wash coat into the active catalyst: preferred alloys contain 15-25% chromium, 4-6% aluminum, 0.3-1% yttrium, cerium, and/or 1-3% cobalt, 0-0.5% carbon, balance iron and the usual impurities.
In the present invention it is preferred that the catalyst is made by forming a gauze, mesh, or pad of wires of a iron/aluminium alloy, applying a wash coat of alumina, ceria, zirconia, or lanthana for example as described in GB-A-1568861, and then applying a dispersion containing the active oxides composition ion or a solution of compounds decomposable to the active oxides. The wash coat is preferably applied to the alloy after effecting surface oxidation of the alloy by calcining the alloy in air at e.g. 1000xc2x0 C. The wash coat is preferably applied as a sol and, where an alumina wash coat is employed, preferably also contains yttria and/or ceria. The coated gauze, mesh, or pad is then subjected to the calcination in air at high temperature to reduce the amount of free cobalt oxides. At the same time this calcination will effect some sintering between adjacent coated wires to bond the gauze, mesh or pad into a strong structure at points where adjacent wires filaments contact one another.
During this calcination of the final composition at high temperature to form the desired mixed oxide structure with minimal free cobalt oxides, it is found that the alumina, and lanthana, if used as the wash coat, are present as diffuse layers extending into adjacent components but are thereafter relatively stable so that little further migration occurs during use.
Instead of using a metallic primary support, a pad, mesh, or gauze formed from ceramic, e.g. alpha alumina, fibres or filaments e.g. by weaving may be employed; such a ceramic primary support may have a wash coat secondary support as aforesaid.
Alternatively, instead of using a gauze, mesh or pad, there may be employed a monolithic support in the form of a honeycomb or foam of a ceramic material such as alumina or zirconia, or a monolithic structure formed from an iron/aluminium alloy e.g. as proposed in GB-A-2077136 but not necessarily employed as a random packed bed of units as proposed in GB-A-2077136. Thus monolithic structures may be used with their passages oriented at preset angles to the gas flow direction. Such monolithic supports may again have a wash coat secondary support as described above.
Accordingly the present invention further provides an oxidation catalyst comprising a primary support in the form of a mesh, gauze, pad, or monolith formed from a high temperature iron/aluminium alloy or a mesh, gauze, pad, monolith, or foam of a ceramic material, a secondary support in the form of an alkali-free alumina or lanthana wash coat on said primary support; and, supported on said secondary support, an active coating of oxides of (a) at least one element A selected from rare earths and yttrium, and (b) cobalt, said cobalt and element A being in such proportions that the element A to cobalt atomic ratio is in the range 0.8 to 1.2, at least some of said cobalt and element A oxides being present as a mixed oxide phase with less than 30, preferably less than 25%, of the cobalt (by atoms) being present as free cobalt oxides.
Where a ceramic honeycomb or foam is employed, it may itself be formed from the catalytic composition, thus avoiding the need for a separate support material.
The catalysts of the invention, particularly those in the form of gauzes, meshes, or pads, may be used as a direct replacement for the conventional precious metal catalysts with essentially no modification to the ammonia oxidation process, except of course the conventional precious metal trap arrangements can be eliminated. In the oxidation of ammonia to nitric oxide for the manufacture of nitric acid, the oxidation process may be operated at temperatures of 800-1000xc2x0 C., particularly 850-950xc2x0 C., pressures of 1 to 15 bar abs., with ammonia in air concentrations of 5-15%, often about 10%, by volume.
In addition for use for ammonia oxidation reactions, the catalysts may also be of use for other oxidations.
The invention is illustrated by the following examples.